netifd: updated IPv6 prefix delegation
* Added support for prefix classes
* Various bugfixes

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@36771 3c298f89-4303-0410-b956-a3cf2f4a3e73
6relayd: Better compatibility with misbheaving DHCPv6 clients
* Make the "best" address / prefix the first one being announced
* Only add the "best" address to the hosts-file

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@36772 3c298f89-4303-0410-b956-a3cf2f4a3e73